This invention relates to methods and apparatus which regulate the temperature of multiple integrated circuit modules by conducting heat thru a pressed joint with each module, one module at a time.
In the prior art, many electromechanical assemblies have been disclosed in which heat flows between an int grated circuit chip and a temperature r gulatlng unit along a thermal conduction path which includes one or more joints. In the case where the components of a joint are rigidly fused togeth r (such as by a solder), then the task of taking the joint apart in order to replace a chip is made difficult. Consequently, fused joints with chips are not suitable in assemblies where the chips are frequently replaced, such as assemblies that test hundreds of chips sequentially.
On the other hand, in the case where a joint consists of two components that are merely pressed together, then the thermal resistance through the joint is increased. This higher resistance occurs because the surfaces of the two components that are pressed together are never perfectly flat, and thus microscopic air gaps exist between the surfaces.
To reduce the thermal resistance through a pressed joint, thermal greases and metal pastes have been developed. When a layer of these materials is placed in the joint between two components that are pressed together, then the microscopic air gaps between the components are reduced which in turn reduces thermal resistance through the joint. Examples of such thermal greases and metal pastes are described in U.S. Pat. No. 5,056,706 which is entitled xe2x80x9cLiquid Metal Paste for Thermal and Electrical Connectionsxe2x80x9d.
However, one drawback of a thermal grease is that its thermal conductivity is still relatively low, in comparison to the conductivity of a metal. See the above U.S. Pat. No. 5,056,706 at column 2, lines 24-29.
Also, another problem with both the thermal greas and the metal paste is that they stick to the two compon nts which are pr ssed together. If the grease or paste is squeezed b tw en a h at xchanger and a chip that is held in a socket, the sticking force can cause the chip to be pulled-out of the socket when an attempt is made to separate the heat exchanger from the chip.
Further, as the chip is separated from the heat exchanger, a residue portion of the grease or paste remains on the separated components. If the heat exchanger is part of an electromechanical assembly which tests hundreds of integrated circuit chips, then any grease or paste which is retained by a chip must be cleaned off of the chip before the chip can be put into an end product. However, the task of cleaning the residue grease or paste from each chip before the chip is put into an end product adds to the time and cost of producing the end product.
Also in the prior art, another pressed joint is disclosed in U.S. Pat. No. 5,323,294 by W. Layton, et al. entitled xe2x80x9cLiquid Metal Heat Conducting Member and Integrated Circuit Package Incorporating Same.xe2x80x9d In this patent, two components are pressed together with a thin compliant body lying between them which has microscopic voids (like a sponge), and a liquid metal alloy is absorbed by the compliant body and partially fills the voids.
However, a drawback of this joint is that it requires the compliant body as a carrier for the liquid metal, and this compliant body is an extra component which adds to the cost of th joint. Also, when the joint is tak n apart, a portion of the liquid metal can be squeezed out of the compliant body and adhere to the two components that were pressed togeth r; and that is a residue which must be cleaned up.
In addition in the prior art, still another pressed joint is disclosed in U.S. Pat. No. 6,243,944 by J. Tustaniwskyj et al which is entitled xe2x80x9cResidue-Free Method of Assembling And Disassembling A Pressed Joint With Low Thermal Resistancexe2x80x9d. This pressed joint can be between a heat exchanger and an integrated circuit package which contains a chip, where the package has a lid that is made of a first material; the heat exchanger has a face that is made of a second material; and a special type of metal alloy is squeezed between the lid on the package and the face of the heat exchanger.
In particular, the above alloy is limited to one that: a) is in a liquid state at a certain temperature at which the chip is initially contacted, and b) adheres in a solid state, at a lower temperature, to the second material (the heat exchanger) but does not adhere to the first material (the integrated circuit package). Since the alloy is liquid when the chip is tested, microscopic air gaps between the lid of the package and the heat exchanger are reduced. Then when the test is complete, the alloy is solidified at the lower temperature so that the package and the heat exchanger can be separated with all of the alloy adhering to the heat exchanger.
However, the present inventors have discovered that a drawback with the above pressed joint is that if the lid of the package is larger than the face of the heat exchanger, then any excess alloy tends to g t squeez d off of the heat exchanger and onto the lid wh n those two components are pressed together while the alloy is in a liquid state. Further, the present inventors have discovered that the remaining alloy which stays on the heat exchanger tends to oxidize while the alloy is in the liquid state. This oxidizing limits the number of chips which can be tested using a single heat exchanger, because as the alloy oxidizes, its thermal resistance increases. By making the face of the heat exchanger larger than the lid of the package, the excess alloy stays on the heat exchanger, and so a larger amount of alloy needs to oxidize before the effect on thermal resistance becomes significant. However, even the larger amount of alloy still tends to oxidize when in a liquid state because it gets xe2x80x9cstirred upxe2x80x9d as it is pressed against the lid of the package; and this eventually limits the number of chips which can be tested with a single heat exchanger.
Accordingly, a primary object of the present invention is to overcome all of the above-described drawbacks with the pressed joints of the prior art.
One embodiment of the present invention is a method of sequentially regulating the temperature of multiple integrated circuit packages while the chips in the packages are electrically tested. This method begins by providing a heat exchanger which has a face that consists essentially of a malleable metal with a coating of a release agent. Then, the face of the heat exchanger is squeezed against an uneven contact surface on the lid of one selected package, while the malleable metal is in a solid state. This squeezing force causes the solid malleable metal to deform, and thereby conform to the shape of the uneven contact surface. During this squeezing step, and while the malleable metal remains in the solid state, the chip in the selected package is electrically tested. When the test is complete, the face of the heat exchanger is separated from the uneven contact surface of the selected package. Then the above squeezing, testing, and separating steps are repeated on each of the remaining packages.
One function which the malleable metal performs is that when it is pressed by just a small force against the contact surface, the malleable metal deforms; and that reduces microscopic air gaps between the malleable metal and the contact surface. This in turn lowers the thermal resistance between the malleable metal and the contact surface. At the same time, the release agent prevents th mall abl metal from sticking to the contact surface; and so th malleabl metal can be easily separated from the contact surface.
Another function which the malleable metal performs is that it remains solid throughout the squeezing and testing steps; and thus the malleable metal cannot move like a liquid, from the face of the heat exchanger to the integrated circuit package when those two components are squeezed together. Also, the malleable metal is much more resistant to being oxidized in the solid state than it would be in a liquid state; and consequently, the thermal resistance of the malleable metal stays essentially constant over many squeezing, testing, and separating cycles.